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Muscular dystrophies are an inherited group of primary diseases of muscle, characterized pathologically by muscle fiber degeneration and clinically by progressive muscle weakness. Pathologic, clinical and genetic criteria have been used as the basis for their classification.

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Duchenne/Becker Muscular Dystrophies (Dystrophinopathies)

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Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) are progressive myopathies, inherited as X-linked recessive traits. DMD is the most common form of muscular dystrophy, with an incidence of about 1 in 3300 live male births and a prevalence rate in the total population of about 3 per 100,000. BMD has a similar presentation but a relatively milder clinical course. The reported incidence of BMD has varied from about 1 in 18,000 to 1 in 31,000 male births. In addition, there is an intermediate group of patients with either mild DMD or severe BMD, who are known as outliers. It is now well known that all 3 types of muscular dystrophy are allelic, resulting from dystrophin deficiency due to mutations of a single gene, called the dystrophin gene.1

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Other dystrophinopathies, occurring at a lower incidence, include:

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  • manifesting DMD/BMD carrier females,
  • X-linked dilated cardiomyopathy, and
  • muscle cramps with myoglobinuria.

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Clinical Presentation

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Great heterogeneity in the clinical features and course of the various dystrophinopathies has been observed, creating a spectrum ranging from very mild to very severe presentations. The severe end of the spectrum includes DMD, BMD the outliers or intermediate phenotype in which skeletal muscle is primarily affected, and X-linked dilated cardiomyopathy in which the heart is the organ primarily affected. Females who carry DMD/BMD can be totally asymptomatic or can manifest mild to severe symptoms.

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Clinically, the distinction between DMD and BMD is made by the age of wheelchair confinement, which is less than 13 years in DMD and beyond 16 years in BMD. Patients who become wheelchair-bound between 13 years and 16 years are classified as outliers or as exhibiting an intermediate presentation. The outlier group could be classified clinically as having either mild DMD or severe BMD.

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Duchenne Muscular Dystrophy
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In children with Duchenne muscular dystrophy (DMD), although there is histologic and laboratory evidence of myopathy from birth, the onset of weakness usually occurs between 2 and 3 years of age; it may be delayed and become apparent after the age of 3 years, but almost all patients with DMD become symptomatic before age 5 years. The child usually has difficulty with running, jumping, going up steps, and other similar activities; an unusual waddling gait, lumbar lordosis, and calf enlargement are usually observed. Muscular weakness is symmetrical and selectively affects proximal limb muscles before distal and the lower extremities before the upper. Early on, the patient may complain of leg pains. Jumping and running are almost impossible in most cases, and, in arising from the floor, affected boys use hand support ...

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